1. Field of the Invention
This invention broadly relates to an improved method of preparing a fluffed pulp exhibiting enhanced dry compression and liquid wicking and retention characteristics. The invention further relates to a fluff pulp sheet product, to the fluffed pulp made using the sheet product and to absorbent articles made using the fluffed pulp.
2. Description of Related Art
Absorbent articles such as disposable diapers, sanitary napkins, and the like, represent one of the major applications for chemically-treated pulp (chemical pulp).
Chemical pulp is prepared by chemically treating cellulosic materials, such as softwoods and hardwoods, to remove their lignin fraction and produce a cellulosic pulp suitable for making paper and related non-woven products. Foremost among the chemical processes are the well-known Kraft and sulfite pulping processes. In the Kraft pulping process, a cellulosic source such as wood chips is digested with an alkaline pulping liquor containing sodium hydroxide and sodium sulfide; while the sulfite process, as the name implies, employs a sulfurous acid solution of an alkali or alkaline earth metal sulfite to effect lignin removal. All known processes also generally rely on some type of post-digestion bleaching to obtain additional lignin removal, and increase the whiteness and brightness of the pulp to enhance commercial acceptance. To produce a pulp product having a sufficient whiteness and brightness for making non-woven mats, the lignin content of the pulp generally is reduced to below about 10 weight percent.
Chemical pulp processed by dry defibration for incorporation into absorbent products is called fluffed pulp. Fluffed pulp is often marketed in the form of roll pulp, but also can be sold in sheet form as bales (hereinafter both referred to a dried fluff pulp sheets). Conventional fluff pulp sheet is manufactured, following the chemical pulping operation, by forming the pulp into a sheet or non-woven mat by any one of several well-known wet-forming processes typified by the conventional Fourdrinier process. In a first step, bleached chemical pulp is deposited upon a screen (or "wire") to form a mat or web of pulp fiber. This step, known in its initial stage as formation, is usually accomplished by passing an aqueous dispersion of a low concentration of pulp (e.g., 0.5% to 1% by weight solids is typical) over the screen. This screen, assisted in certain situations by vacuum or suction, increases the consistency of the mat or web to approximately 20 to 35 weight percent solids.
In a second step, the mat or web is compressed or squeezed in a "press section" to remove additional water. This is usually accomplished by felt presses, a series of rollers each having a felted band for contact with the mat or web. These presses remove additional free water and some capillary water, thus resulting in an increase in consistency of the mat or web to a range of about 30 to 60 weight percent. As is well known, in making fluff pulp sheet, less pressure is applied in this portion of the process than normally would be encountered in conventional paper-making, thus less water is removed in this section. Less pressing is done so as to facilitate subsequent comminution of the fluff pulp sheet to the defibrated fluffed pulp.
Following the press section, the pulp sheet is then dried in a dryer section. Because a reduced amount of water was removed in the press section, more moisture must be removed from the sheet in the dryer section than generally is necessary in paper-making. In the drier section, the remaining water content of the pulp sheet is reduced to obtain a pulp consistency which typically ranges between about 88 to 97 weight percent (3 to 12 weight percent moisture), more usually between 90 to 94 weight percent (6 to 10 weight percent moisture).
For use in absorbent products such as diapers, the sheets formed in this manner are thereafter comminuted using a variety of known techniques and machines such as hammermills. The comminuted pulp is referred to hereinafter as fluffed pulp. The fluffed pulp fibers can then be used to form an absorbent product.
While absorbent articles made using conventionally produced fluffed pulp have been accepted commercially, common disadvantages associated with the use of standard chemical fluffed pulp include its limited dry compression characteristics and its limited liquid wicking and water retention property. One consequence of limited dry compression is that the energy requirement for making densified absorbent products is higher than if the pulp exhibited a greater degree of dry compression. Poor wicking property reduces the pulp's re-wetting ability, i.e. its ability to retain moisture when subjected to several doses (insults) of liquid, and impedes the distribution of fluid through a mat of the fibers.
U.S. Pat. No. 4,065,347 describes making fluffed pulp from bales or blocks of an unwashed mechanical pulp, made by defibration of wood chips in a defibrator or refiner.
U.S. Pat. No. 4,081,316 relates to a purportedly improved fluffed pulp produced by a method comprising the steps of mixing ground wood (mechanical pulp) with a portion of a beaten chemical pulp, mechanically dewatering the wet mixture, coarse-defibrating the dewater fibers, drying in a flash drying step, finish defibrating and finish drying.
U.S. Pat. No. 5,547,541 describes a process that purportedly produces a fluff pulp sheet which allows for an improved densification of fluffed fibers. The process requires the addition of a chemical densifying agent to the pulp fibers following sheet formation. The addition of such chemicals to the pulp adds significantly to the cost of the pulp fibers and may affect liquid transport and liquid retention characteristics of the final pulp fibers in unanticipated ways.
As a result, a method of treating pulp fibers to improve their densification (compaction) properties, which method does not involve a chemical treatment, would represent a significant improvement in the art of making a fluffed pulp.
Accordingly, it is an object of the present invention to provide a method that produces a fluffed pulp of improved compaction characteristics.
It is another object of the present invention to provide a method of processing chemical pulp that produces a fluffed pulp of improved wicking, liquid retention and liquid distribution characteristics.
The present invention relates to these and other objects which will become readily apparent from a reading of the following description of the invention and accompanying drawings.